1. Field of the Invention
The invention relates to a discharge lamp. The invention relates especially to a discharge lamp of the short arc type that is used as a light source for a projection device, a photochemical reaction device, and an inspection device.
2. Description of Related Art
Discharge lamps can be classified into different lamp types with respect to the emission substance, the distance between the electrodes, and the internal pressure of the arc tube. With respect to the type of lamp classified by its emission substance, there are xenon lamps, with xenon gas as the emission substance, mercury lamps with mercury as the emission substance, metal halide lamps with rare earth metals besides mercury as the emission substance, and the like. With regard to the type of lamp classified by the distance between the electrodes, there are discharge lamps of the short arc type, and discharge lamps of the long arc type. With respect to the type of lamp classified by the vapor pressure within the arc tube, there are low pressure discharge lamps, high pressure discharge lamps, ultra high pressure discharge lamps, and the like.
In a high pressure mercury lamp of the short arc type, there are tungsten electrodes with a distance from roughly 2 mm to 12 mm in an arc tube made of silica glass with a high thermal stability temperature, and the arc tube is filled with a gas, such as mercury, argon, or the like, as the emission substance with a vapor pressure during operation of 105 Pa to 107 Pa. Since it is advantageous in that the distance between the electrodes is short and high radiance can be obtained in this high pressure mercury discharge lamp of the short arc type, it is conventionally often used as a light source for exposure in lithography.
On the other hand, recently it has been considered not only as the light source for exposing a semiconductor wafer, but also as the light source for exposure of a liquid crystal substrate, especially a liquid crystal substrate used for a liquid crystal display with a large area. Also, with respect to an increase of throughput in the production process, there is a high demand for increasing the output power of a lamp used as a light source.
When the output power of the discharge lamp is increased, the nominal power consumption is also increased. The value of the current flowing into the discharge lamp generally increases even if it depends on the computed data of the current and the voltage.
With respect to the electrodes, especially the anode, during operation using a direct current the amount of electron bombardment increases. This leads to the disadvantage in which the electrodes' temperature increases slightly causing melting. In a discharge lamp which is positioned in the vertical direction, the electrode, not limited to an anode, is located at the top and is influenced by the heat convection in the arc tube or the like. The electrode receives heat from the arc more intensely, and is, thus, subjected to a temperature increase causing it to melt.
If the electrode, especially its tip area, melts, the arc becomes disadvantageously unstable, and, moreover, the material comprising the electrode vaporizes and adheres to the inside surface of the arc tube causing radiation output to decrease.
Such phenomenon is not limited to a high pressure mercury discharge lamp of the short arc type, but is disadvantageously and generally occurred in the case of an increase of the output power of a discharge lamp. Hence, conventionally, there are an arrangement and a process in which an air cooling device and compressed air cooling is carried out outside the discharge lamp. In a discharge lamp with a greater output power, a so-called discharge lamp of the water cooling type has been proposed, for example, by Japanese Patent No. 3075094, or U.S. Pat. No. 5,633,556, in which within the electrode there is a cooling water passage allowing cooling water to flow.
In the process where increasing the output power of the discharge lamp is possible by using an air cooling device located outside the discharge lamp to provide forced air cooling, the current that can be introduced into the discharge lamp however still has a boundary value or upper limit. Therefore, it is difficult to increase the output power even with external air cooling. This boundary value differs slightly depending on the type of discharge lamp and environment in which the discharge lamp is located. The value of the current supplied to the discharge lamp is roughly 200 A. An increase in the current exceeding this value was not possible in practice.
In the case of a discharge lamp of the water cooling type, water is fed into the electrode and is allowed to flow out. In the vicinity of the discharge lamp there must be a circulation pump, a system for feeding cooling water, and a drain device. As a result of having the cooling system, the discharge lamp is increased in size. A cooling device, which is many times larger than the discharge lamp, is required. The water cooling process may, therefore, indeed be useful for special applications, but has only little general utility for a discharge lamp. Particularly, it cannot not be maintained especially and suitably for a light source of an exposure device for lithography used in a clean room.
Moreover, in a process depending only on a forced cooling device, there is an area within the arc tube with an especially low temperature where a filler, such as mercury or the like, collects in the unvaporized state. In such a case, the given operating pressure of the discharge lamp is not obtained, and neither the desired amount of radiant light nor the desired radiance is obtained. In the case where the temperature within the arc tube has dropped unduly, the arc formed between the electrodes becomes unstable, thereby causing vaporization and flickering of the discharge lamp.